1. Field of the Invention
The present invention relates to a process for manufacturing a multi-layer glass ceramic substrate, in which the firing shrinkage factors of the layers are controlled to eliminate the problem of a delamination between layers.
Information processors (CPU's) are designed to have a small size and a bulk storage capacity and to rapidly process a large quantity of information. Also, semiconductor devices are assembled with a high packaging density, and as a result, are equipped with a large number of semiconductor elements such as LSI's and VLSI's densely arranged thereon.
As circuit boards carrying these semiconductor elements in these devices, initially multi-layer printed circuit boards comprising, for example, a glass epoxy were used. Recently, however, the generated heat per semiconductor chip has become more than 10 W, as the integration density of a semiconductor device has increased and accordingly, the multi-layer printed circuit board has been replaced by a ceramic substrate having a superior heat resistance.
2. Description of the Related Art
Initially, alumina (Al.sub.2 O.sub.3) ceramics were employed as materials for ceramic substrates, and tungsten (W) was used as a metal material for forming patterns of wiring. The signal frequency processed by or transmitted through such a device reaches frequencies on the order of MHz, in line with the increased speed of information processing and optical communication, is already in practical use and as a result, the use of ceramic substrates having a low loss and conductive materials having a low electrical resistance is required.
In this respect, although copper (Cu) is a preferred conductive material having a low electrical resistance, it has a low melting point on the order of 1084.degree. C., and accordingly, copper cannot be used as a material for distributing wires of a multi-layer circuit board comprised of alumina ceramics, the green sheet of which requires a firing temperature of more than 1400.degree. C.
Accordingly, glass ceramics requiring a low firing temperature are used.
More specifically, glass ceramics require a low firing temperature on the order of not more than 1000.degree. C., and this allows the use of metallic materials such as gold (Au) and Cu having good electrical conductivities.
Typical examples of glass ceramics are composed of alumina powder and borosilicate glasses. The use of these glass ceramics is effective for the following reasons. The dielectric constant of the borosilicate glass is low, on the order of about 4, and correspondingly, the combined dielectric constant of the resulting composite material ranges from 5 to 7, which is lower than that (about 10) of alumina ceramics. This accordingly allows a shortening of the signal delay time.
Nevertheless, it has become necessary to further reduce the combined dielectric constant of substrates, in proportion to the increased speed of the signal transmission, and accordingly, a part of the borosilicate glass is replaced with hollow silica powder.
Namely, the use of the hollow silica powder permits a further reduction of the combined dielectric constant, since the dielectric constant of air is 1.
The mechanical strengths of glass ceramics, however, are not satisfactory; for example, the bending strength thereof is on the order of 150 to 200 MPa, which is lower than that of alumina ceramics. The mechanical strength of the resulting glass ceramics would be further reduced if a part of the borosilicate glass component were replaced by the hollow silica powder, and accordingly, the glass ceramics would be easily damaged.
To solve this problem, i.e., to improve the mechanical strength of the resulting multi-layer glass ceramic substrate, a method has been proposed which comprises forming signal layers, which are elements of the multi-layer glass ceramic substrate and on which patterns of signal lines are formed, from a glass ceramic containing hollow silica powder, and forming other layers of the substrate, such as the uppermost and lowermost layers which do not affect the quality of a signal, from a glass ceramic free of the hollow silica powder.
In this case, however, the multi-layer glass ceramic substrate comprises layers having different compositions and this leads to the occurrence of a delamination between the signal layers and the upper and lower layers of the substrate during the firing of the corresponding green sheets, due to the differences in the firing shrinkage factors of these layers having different compositions.
Accordingly, an object of the present invention is to solve this problem.